Add display icon in "Insert media" post edition popin

Add an icon display for OAM files in "Insert Media" popin on post editing:

![capture decran 2013-11-22 a 16 53 10](https://f.cloud.github.com/assets/103900/1601771/4d894892-538e-11e3-93f4-20b97513e1bf.png)